Gas absorption analysis using, for example, FTIR (Fourier transform infrared) spectroscopy employs a White type multireflection cell having a multireflection mechanism that is configured to include multiple mirrors in order to decrease the required volume of measurement target gas as well as increasing the light path length of light passing through the measurement target gas (see Non-Patent Literature 1).
A multireflection mechanism MR illustrated in FIG. 9 is arranged inside a cell main body (not illustrated in FIG. 9) into which measurement target gas is introduced. Also, the multireflection mechanism MR includes: one field mirror 12; a first objective mirror 13 that faces to the field mirror 12 and is provided on a light incident side in the multireflection mechanism MR; and a second objective mirror 14 that faces to the field mirror 12 and is provided on a light emitting side in the multireflection mechanism MR.
Into such a multireflection mechanism MR, light having a predetermined spread with respect to a light axis is introduced so as to be first reflected by the first objective mirror 13. After that, the introduced light is repeatedly reflected between the field mirror 12 and the first or second objective mirror 13 or 14. The light repeatedly reflected in the multireflection mechanism MR is finally emitted outward from the emitting side that is set on the side opposite to the light incident side in the multireflection mechanism MR.
Meanwhile, when desiring to measure a time change of a component of measurement target gas, such as to analyze exhaust gas of an engine, it is demanded to improve responsiveness by decreasing the volume of the cell main body to minimize a time to replace the measurement target gas to be introduced.
When decreasing the volume of the cell main body in order to respond to the demand, the interval between the opposite mirrors is inevitably shortened. For this reason, in order to achieve measurement accuracy comparable to conventional one, it is necessary to increase the number of times of light reflection in the multireflection mechanism MR to keep a light path length comparable to a conventional length.
However, when increasing the number of times of reflection in the multireflection mechanism MR, reflection points are more densely concentrated in the vicinity of the outer edge of the field mirror 12 than the central part. As a result, reflection points are overlapped near an emitting port OP in the vicinity of the outer edge, and part of the light may be emitted outward from the multireflection mechanism MR before reaching a prescribed number of times of reflection (see FIG. 8). For this reason, the absorbance of the measurement target gas cannot be accurately measured.